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The present invention relates to fiber optic gyroscopes (hereinafter referred to as xe2x80x9cFOGxe2x80x9d), and more particularly, to FOG coils constructed and arranged to reduce the rate of absorption of ambient moisture.
It is well known that moisture can degrade the performance and reliability of optical fibers. Micro-cracks in the glass fiber can propagate in the presence of ambient moisture which in turn can change the optical properties of the fiber and potentially lead to premature failure. The effects of moisture depends on many factors, including environmental conditions, the nature of the fiber manufacturing process, etc. Since the amount of moisture can change with environmental conditions (e.g., temperature), the optical properties of the fiber can change, often unpredictably, as a function of those conditions.
Fiber manufacturers typically apply an acrylate (or other similar polymeric material) protective coating directly to the outer surface of the glass fiber to mitigate the effects of ambient moisture. Such a coating creates a barrier to moisture, provides some level of abrasion resistance and permits handling since bare fiber is very fragile. While the coating may environmentally protect the fiber, the coating itself may absorb a significant amount of moisture. This phenomenon has been observed during bake-out procedures, i.e., when the fiber is subjected to controlled high temperature environments, for an extended amount of time. During bake-out procedures, coated optical fibers experience a significant weight change (e.g., 12 percent or more), and hence a diametrical change, implying that the coating surrenders a significant amount of captured moisture while in the high temperature environment.
A coil of optical fiber is a critical component in an Interferometric Fiber Optic Gyroscope (IFOG, or more simply, FOG). A FOG is a device used to measure the rate of rotation of a vehicle or other platform to which the FOG is attached. The FOG typically includes a coil of optical fiber disposed about an axis of rotation. A light source transmits light into each end of the optical fiber, so that two light transmissions propagate through the optical fiber in counter-rotating directions. Detection circuitry receives the light transmissions as they emerge from the ends of the optical fiber and measures the relative phase relationship of the light. The phase relationship of the two light transmissions is related to the angular rotation of the FOG coil about the axis of rotation, and may be used to derive an output signal that is indicative of the rate of rotation of the FOG coil.
An important parameter associated with a FOG, commonly referred to as the xe2x80x9cscale factor,xe2x80x9d defines and quantifies the relationship between the actual rate of rotation of the FOG to the output signal of the FOG device (e.g., number of output pulses per arc-second of rotation). Variations in the FOG scale factor tend to decrease the accuracy of the FOG. The optical diameter of the fiber optic coil directly influences the scale factor of the FOG, so any external influences that could affect the optical diameter will also affect the scale factor. The optical diameter is closely related to the physical diameter of the coil, so any change in the physical diameter of the coil can effect the scale factor of the FOG. Therefore, moisture absorption by the fiber jacket directly affects the overall fiber diameter and hence the resultant scale factor of the FOG.
Epoxy materials are often applied about and between layers of optical fibers in the coils to provide physical stability of the winding layers, and to maintain the coil geometry over environmental stresses. Such epoxy materials are known to be amorphous with inhomogeneities that are commensurate with the size of water molecules, so as to permit the transport of water molecules through capillary action. These epoxy materials are thus hygroscopic, and if the stabilizing epoxy material absorbs a significant amount of moisture, the epoxy material can expand and/or deform, thus changing the coil geometry and affecting the performance of the FOG.
In one aspect, a method of applying a moisture barrier seal to a fiber optic coil comprises mounting a fiber optic coil in a vacuum deposition chamber, so as to expose a large exterior surface area of the fiber optic coil to an interior portion of the deposition chamber. The method further includes reducing the air pressure within the chamber to a value that is less than ambient pressure outside of the chamber. The method further includes introducing a vapor form of a non-porous material into the chamber. The vapor form of the non-porous material changes into a solid state upon contact with the fiber optic coil, so as to form a conformal coat on the fiber optic coil.
In one embodiment, the method further includes evacuating at least some of the air within the chamber (i.e., removing air from inside the chamber) so as to reduce the air pressure within the chamber.
In another embodiment, the method further includes heating the non-porous material until it converts into a gaseous, vapor form.
In another embodiment, the method further includes introducing parylene vapor into the vacuum deposition chamber.
In another embodiment, the method further includes heating a predetermined quantity of parylene material until the parylene material transforms into a gaseous, parylene vapor.
In another embodiment, the method further includes reducing the air pressure within the chamber to a predetermined value less than the ambient air pressure, wherein the predetermined value is a nominal vacuum deposition pressure.
In another aspect, a system for applying a moisture barrier seal to a fiber optic coil comprises mounting means for mounting a fiber optic coil in a vacuum deposition chamber, so as to expose a large exterior surface area of the fiber optic coil to an interior portion of the deposition chamber. The system further includes means for reducing air pressure within the chamber to a value less than ambient pressure outside of the chamber. The system also includes means for introducing a vapor form of a non-porous material into the chamber. The vapor form of the non-porous material changes into a solid state upon contact with the fiber optic coil, so as to form a conformal coat on the fiber optic coil.
In one embodiment of the system, the mounting means further includes mounting provisions constructed and arranged so as to expose a maximum amount of the exterior surface of the fiber optic coil to an environment within the deposition chamber.
In another embodiment of the system, the means for reducing air pressure further includes a vacuum pump for removing at least some air from within the deposition chamber.
In another embodiment, the vacuum pump reduces air pressure within the chamber to a predetermined value less than ambient pressure. The predetermined value is a nominal vacuum deposition pressure for applying the non-porous material vapor to the fiber coil.
In another embodiment, the means for introducing a vapor form of a non-porous material further includes a vapor generator for heating a predetermined amount of the non-porous material until the non-porous material sublimes into a vapor form. In one embodiment, the non-porous material includes parylene.
In another embodiment, the fiber optic coil remains in the chamber for a predetermined amount of time, surrounded by the vapor form of the non-porous material at the air pressure value less than ambient pressure.
In another aspect, a system for applying a moisture barrier seal to a fiber optic coil comprises a deposition chamber, a vacuum pump and a vapor generator. The deposition chamber has an access hatch for transferring the fiber optic coil into or out of the deposition chamber, a vacuum port for transferring air into or out of the deposition chamber, and an input port for transferring deposition material into or out of the deposition chamber. The vacuum pump is attached to the vacuum port, and pumps air out of the deposition chamber, so as to reduce air pressure within the chamber to a value less than ambient pressure outside of the chamber. The vapor generator is attached to the input port, and introduces a vapor form of a non-porous material into the chamber. The vapor form of the non-porous material changes into a solid state form upon contact with the fiber optic coil, so as to create a conformal coat on the fiber optic coil.
In another embodiment of the invention, the deposition chamber further includes mounting provisions for mounting the fiber optic coil within the deposition chamber. The mounting provisions may include a bracket, pedestal or other similar assembly known in the art for securing the fiber optic coil. The mounting provisions are constructed and arranged so as to expose a large exterior surface of the fiber optic coil to an interior portion of the deposition chamber.
In another embodiment of the invention, the vacuum pump reduces air pressure within the chamber to a predetermined value that is less than ambient pressure. This predetermined value of pressure in the chamber is a optimal vacuum deposition pressure, i.e., a pressure that allows the best deposition of the non-porous material on the exterior surface of the optical fiber coil, without damaging the coil.
In another embodiment, the vapor generator heats a predetermined amount of the non-porous material until the non-porous material sublimes into a vapor form. In one embodiment, the non-porous material includes parylene.
In another embodiment of the system, the fiber optic coil remains in the chamber for a predetermined amount of time, surrounded by the vapor form of the non-porous material at the air pressure value less than ambient pressure. Although material quantity and process time defines the thickness and hence performance of the coating as a moisture barrier, it is important that the coating is not too thick. Thick coatings might adversely affect the performance of the FOG due to differential thermal expansions that can induce stress on the optical fiber.